Photothermal recycling of waste polyolefin plastics into liquid fuels with high selectivity under solvent-free conditions

The widespread use of polyolefin plastics in modern societies generates huge amounts of plastic waste. With a view toward sustainability, researchers are now seeking novel and low-cost strategies for recycling and valorizing polyolefin plastics. Herein, we report the successful development of a photothermal catalytic recycling system for transforming polyolefin plastics into liquid/waxy fuels under concentrated sunlight or xenon lamp irradiation. Photothermal heating of a Ru/TiO2 catalyst to 200–300 °C in the presence of polyolefin plastics results in intimate catalyst-plastic contact and controllable hydrogenolysis of C-C and C-H bonds in the polymer chains (mediated by Ru sites). By optimizing the reaction temperature and pressure, the complete conversion of waste polyolefins into valuable liquid fuels (86% gasoline- and diesel-range hydrocarbons, C5-C21) is possible in short periods (3 h). This work demonstrates a simple and efficient strategy for recycling waste polyolefin plastics using abundant solar energy.

Measurements were performed in triplicate to ensure consistency of the results.
Measurement of molecular weight. The weight-average molecular weight (Mw), number-average molecular weight (Mn), and molecular weight distributions (Đ = Mw/Mn) of the different polymers were determined by high-temperature gel permeation chromatography (GPC) on an Agilent PL-GPC-220 instrument equipped with three detectors (a two-angle laser light scattering detector, a refractive index detector, and a viscosity detector) and three PL-Gel Mixed B columns. The samples were dissolved in TCB containing 0.01 wt.% BHT and heated at 150 °C for at least 2 h. Elution of columns used a 1 mL/min flow of TCB (with BHT) at 150 °C. Before the sample measurements, the GPC system was calibrated using monomodal, linear polyethylene standards (Varian).
Gas chromatography. Gaseous products of the photothermal polyolefin conversion experiments were analyzed on a GC-2014C gas chromatograph (Shimadzu Co., Japan) equipped with three channels. The first channel analyzed hydrocarbons (C1-C7) using a HP PLOT Al2O3 column with He as the carrier gas and a flame ionization detector (FID).
The second channel analyzed CO2, N2, Ar, O2, CH4, and CO with a combination of micropacket Haysep Q, H-N, and Molsieve 13× columns using He as the carrier gas and a thermal conductivity detector (TCD). The third channel analyzed H2 using a micropacket HayeSep Q and Molsieve 5 Å column with N2 as the carrier gas and a TCD detector. The gaseous products were quantified by an external standard method based on methane. A Shimadzu GC-2014 gas chromatograph (Shimadzu Co., Japan) equipped with a HP-5 column and FID with N2 as carrier was used for the analysis of higher hydrocarbons (C5-C37).

Calculation of polyolefin degradation percentage
The polyolefin degradation percentage during ambient-pressure photothermal experiments was calculated using the equation: Where Mi is the input mass of Ru/TiO2 and polyolefin substrate, Mr is the mass of Ru/TiO2 and unreacted polymer residue, Ms is the input mass of polyolefin substrate.
Standard deviation calculated from two samples was utilized in the error bar.
Analysis of liquid/waxy products. 1  The temperature of the LDPE and Ru/TiO2 catalyst mixture showed a linear correlation with the light intensity of the Xe lamp, thus allowing accurate control of the photothermal reaction temperature by adjustment of the light intensity. In addition, we could also adjust the reaction temperature by auxiliary heating with a heating element or cooling using circulating water. Based on the UV-Vis diffuse reflectance spectra for TiO2 ( Supplementary Fig. 1) and LDPE (Fig. 3d), 365 nm light should be able to induce photocatalytic activity in TiO2 but will not be absorbed by LDPE. The data in Supplementary Fig. 16 exclude the possibility that a photocatalytic effect involving TiO2 contributed to LDPE degradation since data collected under thermal and thermal + 365 nm irradiation were nearly identical. Furthermore, Ru nanoparticles show a maximum intensity of the generated electric field at 368 nm, very close to the excitation wavelength of 365 nm. Since the molecular weight distributions in the above plot were similar under the thermal and thermal + 365 nm regimes, any promotion due to LSPR effect involving the Ru nanoparticles was negligible. As shown in Supplementary Fig. 18, CH4 production was greatly suppressed after the introduction of UV light. Previous studies of LDPE thermal decomposition have shown that CH4 generation results from direct terminal C-C scission (methane produced via a surface cascade of consecutive C-C scissions can be nearly ignored without any catalyst at this reaction temperature) 5 . Lower production rates of CH4 under UV irradiation indicated preferential internal C-C scission instead of terminal C-C scission, implying that internal C-C bonds were more easily weakened by UV irradiation, and thus changing the cleavage mode 6 . Thus, quicker degradation of LDPE and higher selectivity of liquid fuels can be achieved using the photothermal reaction. Notable degradation of LDPE was observed over the Ru powder, verifying the remarkable C-C bond cleavage ability of metallic Ru sites ( Supplementary Fig. 20a). The difference in performance between the Ru/TiO2 catalyst and Ru powder is ascribed to their different H2 adsorption ability and activation properties arising from the distinct specific surface area and Ru nanoparticle size. The action of Ru sites in promoting C-C bond cleavage during polyolefin degradation has previously been reported in studies using Ru/C 8 , Ru-modified zeolite 9 , and Ru/CeO2 10 catalysts. TiO2 showed poor performance for LDPE degradation at hydrogen partial pressures of 1 bar H2/Ar (v/v = 30/70) 7 . As shown in Supplementary Fig. 20b, Benefiting from the efficient hydrogenolysis and synergistic effect of UV-Vis-NIR light, the photothermal recycling system showed enhanced LDPE recycling performance compared to thermal recycling at each temperature studied. The extent of LDPE degradation increases as the reaction temperature increased, with 100% LDPE degradation achieved during photothermal recycling at 220 °C. Results confirmed the high efficiency of the photothermal LDPE recycling system. The LDPE degradation percentages determined under concentrated sunlight were slightly lower than those obtained using the Xe lamp due to slight fluctuations in reaction temperature and light intensity between sunny and cloudy conditions outdoors.

Supplementary Fig. 47 | Flow diagram of industrial polyethylene hydrogenolysis based on the simulation through Aspen Plus software.
We attempted to perform a simple technoeconomic analysis of industrial polyethylene hydrogenolysis using Aspen Plus simulation software. Here, the industrial hydrogenolysis process of polyethylene was simulated by treating 8640 tons of polyethylene per year. In a typical process, the waste polyethylene, catalyst, and H2 are pumped into the reactor (B1) with 100% of solid conversion to produce the C1 to C21 hydrocarbons, followed by the separation (B2) of gases including H2 and C1 to C4 (6) and the nongaseous mixture of catalyst and C5 to C21 hydrocarbons (7). Subsequently, the gas components pass through separator 2 (B3) to isolate the light hydrocarbons (C1 to C4) (8), with excess H2 being recycled and fed into the reactor for the next batch of reaction (3). A solid-liquid filter (B4) was used in another product line to separate the liquid C5 to C21 hydrocarbons (9), and the residual solid catalyst was recovered and reused (4). In addition, the liquids were transported to a rectifying tower (B5) to vaporize the gasoline product (C5 to C12) (10) and isolate the diesel product (C13 to C22) (11). The energy consumed by each equipment and the total process was calculated on the basis of polyethylene hydrogenolysis by Ru/TiO2. Compared to the two separators and the rectifying tower, the reactor consumes most of the energy (347.9 kW/h), accounting for 90.0 % of the energy input required for the whole process (Supplementary Table 3). Such energy consumption will be significantly lowered if the reactor is powered by solar energy using concentrated solar power technology 13 , resulting in substantial cost reductions. Concentrated solar power technology has been reported commercially feasible in supercritical water gasification integrated with Fischer-Tropsch synthesis 14 , liquid hydrocarbon fuels from CO2 and H2O 15 , solar hydrogen production 13 , liquid fuel and hydrogen coproduction 16 . In addition, molten salt thermal storage systems based on a tower design can achieve 24 h operation in the summertime 13 . Hence, our simple analysis suggests the commercial feasibility of photothermal polyolefin recycling due to the energy savings and rapid technological development of concentrated solar power technology. The capital investment cost of building a photothermal polyolefin recycling was not considered here.

Supplementary Tables
Supplementary Table 1  In the photothermal recycling experiments, a substantial amount of waxy products were obtained at a pressure of 10 bar. The selectivity to liquid fuels increased gradually as the pressure increased with the highest C5-C21 selectivity (86%) achieved at 30 bar. The selectivity to liquid fuels decreased with a further elevation of the reaction pressure (40 bar) due to the excessive gasification of liquid fuels. Whilst thermal recycling was capable of completely degrading the LDPE bags, photothermal recycling offered significantly higher selectivity to liquid fuels (86%) compared to thermal recycling (64%) under the same conditions. Results confirmed the superiority of the photothermal recycling system for producing liquid fuels.